The present invention relates generally to gas turbine engines, and, more specifically, to air cooling of hot components therein such as turbine vanes, blades, and shrouds.
A gas turbine engine includes a compressor for compressing air which is suitably mixed with fuel and ignited for generating combustion gases in a combustor disposed in flow communication with a turbine which extracts energy therefrom for powering the compressor and producing output power such as that used for powering an electrical generator. The turbine includes one or more stages of stator nozzles or vanes, rotor blades, and annular shrouds around the turbine blades for maintaining suitably tight clearances therewith.
The turbine vanes, blades, and shrouds are heated by the hot combustion gases and are typically cooled during operation to ensure a useful life thereof. Cooling is typically accomplished by bleeding a portion of the compressed air from the compressor and suitably channeling it through various conventional local circuits in these components for providing cooling thereof. These turbine components typically include conventional film cooling holes therethrough which discharge the cooling air in a boundary layer film for protecting the components from the hot combustion gases. The cooling air mixes with the combustion gases in the turbine and reduces engine efficiency due to thermodynamic and aerodynamic reasons. Furthermore, any compressed air bled from the compressor and not used in generating the combustion gases in the first instance also necessarily reduces efficiency of the engine, and therefore, it is desirable to minimize the amount of cooling air bled from the compressor.
Turbine efficiency increases as the turbine inlet temperature increases but the increasing temperature also requires more effective cooling of the heated components. The art is crowded with various techniques for locally cooling turbine vanes, blades, and shrouds for maximizing cooling effectiveness with a minimum amount of cooling air.
Air cooling circuits of typical gas turbine engines are open-circuits since the cooling air is bled from the compressor and reintroduced into the turbine after cooling the hot components thereof. Closed-circuit cooling of gas turbine engines is known in the literature and is typically combined with steam turbines in a combined cycle power plant. Steam from the steam turbine is used in a closed-circuit through the hot gas turbine components for cooling thereof without the use of film cooling holes which would reintroduce the fluid into the gas turbine. However, a steam turbine is necessary for producing the cooling steam for the gas turbine engine, and, steam has different heat transfer capabilities than simple air.
In another closed-circuit cooling arrangement for a gas turbine engine, compressor discharge air is extracted from a plenum surrounding the combustor, precooled in a suitable heat exchanger, and then further compressed in a booster compressor to provide the required driving pressure to channel the cooling air through a closed-circuit in turbine blades, with the spent cooling air being returned to the combustor plenum. Significant advantages accrue due to closed-circuit air cooling since the air is not discharged into the turbine through film cooling holes and therefore the associated thermodynamic and aerodynamic losses are eliminated. However, this arrangement is relatively complex and requires the auxiliary booster compressor for being implemented.